Progressive cavity pumps are used for artificial oil lifting operations on wellheads. FIG. 1 illustrates a typical progressive cavity pump system 10 for a wellhead 12. The progressing cavity pump system 10 has a surface drive 20, a drive shaft 30, and a downhole progressive cavity pump 40. At the surface of the well, surface drive 20 has a drive head 22 mounted above wellhead 12 and has an electric or hydraulic motor 24 coupled to drive head 22 by a pulley/belt assembly or gear box 26. Drive head 20 typically includes a stuffing box (not shown), a clamp 28, and a polished rod 29. The stuffing box is used to seal the connection between drive head 22 to drive shaft 30, and the clamp 28 and polished rod 29 are used to transmit the rotation from the drive head 22 to the drive shaft 30.
Downhole, progressive cavity pump 40 installs below the wellhead 20 at a substantial depth (e.g., about 2000 m) in the wellbore. Typically, pump 40 has a single helical-shaped rotor 42 that turns inside a double helical elastomer-lined stator 44. During operation, the stator 44 attached to production tubing string 14 remains stationary, and surface drive 20 coupled to rotor 42 by drive string 30 cause rotor 42 to turn eccentrically in stator 44. As a result, a series of sealed cavities form between stator 44 and rotor 42 and progress from the inlet end to the discharge end of pump 40, which produces a non-pulsating positive displacement flow. Because pump 40 is located at the bottom of the wellbore, which may be several thousand feet deep, pumping oil to the surface requires very high pressure. The drive shaft 30 coupled to the rotor 42 is typically a steel stem having a diameter of approximately 1″ and a length sufficient for the required operations. During pumping, shaft 30 may be wound torsionally several dozen times so that shaft 30 accumulates a substantial amount of energy. In addition, the height of the petroleum column above pump 40 can produce hydraulic energy on drive shaft 30 while pump 40 is producing. This hydraulic energy increases the energy of the twisted shaft 30 because it causes pump 40 to operate as a hydraulic motor, rotating in the same direction as the twisting of drive shaft 30.
If operation of system 10 is stopped due to normal maintenance shutdown, loss of power, or overload, the accumulated energy and pressures on drive shaft 30 will cause shaft 30 to reverse spin or unwind, and this energy is transmitted to surface drive 20 as backspin. Forces generated by the backspin can then damage the surface drive 20, for example, by disintegrating pulleys or the like. To alleviate these effects, a braking system or a backspin retarder is used in surface drive 20 to control of the backspin of drive shaft 30 until the fluid head and wind-up of drive shaft 30 have been reduced to a desired level.
Typical braking systems use a ratchet or free wheel arrangement that allows for two operational modes—either free-turning or braking. For example, such ratchet or free wheel arrangements allow rotation in one direction during normal operation but actuate the braking system when rotation occurs in the opposite direction, referred to as “backspin.” In this way, the braking components are only activated if there is rotation in the opposite direction.
Unfortunately, an originally installed braking system on a wellhead may no longer be capable of performing its original function for any number of reasons. For example, chemical and mechanical wear may damage hoses, connections, seals, etc. of the original wellhead braking system. In addition, surface drive 20 may overload causing wellhead to shut down, which strongly indicates that pump 40 is jammed at the bottom of the well. Such jamming may occur due to swelling of the stator's elastomer components reacting to the petroleum. In addition, intake of sand or other debris can also cause jamming. When jamming occurs and surface drive 20 lacks a torque limiter system (such as a frequency inverter programmed for this purpose), then drive 20 continues rotating shaft 30 and accumulating more energy until drive 20 breaks down due to overload. In this situation, drive 20 can apply many times the nominal torque to drive shaft 30, and the cumulative torque can even exceed the technical specifications for the braking system.